Ftm protocol enhancements for channel delay spread

ABSTRACT

This disclosure provides systems, methods and apparatuses for selecting a guard interval for transmission of orthogonal frequency-division multiplexing (OFDM) symbols on an uplink (UL) channel and on a downlink (DL) channel. In some implementations, a transmitting device and a receiving device can estimate channel delay spread (CDS) information for each other&#39;s transmit channels, exchange the estimated CDS information with each other, and select a guard interval based on the estimated CDS information exchanged with each other. The transmitting device can transmit a number of OFDM symbols separated by the selected guard interval to the receiving device on the UL channel, and the receiving device can transmit a number of OFDM symbols separated by the selected guard interval to the transmitting device on the DL channel.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of co-pending and commonly ownedU.S. Provisional Patent Application No. 62/312,616 entitled “FTMPROTOCOL ENHANCEMENTS FOR CHANNEL DELAY SPREAD” filed on Mar. 24, 2016,the entirety of which is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates generally to wireless networks, and specificallyto selecting a guard interval for transmitting orthogonalfrequency-division multiplexing (OFDM) symbols over a wireless channel.

DESCRIPTION OF THE RELATED TECHNOLOGY

A wireless local area network (WLAN) may be formed by one or more accesspoints (APs) that provide a shared wireless medium for use by a numberof client devices or stations (STAs). Each AP, which may correspond to aBasic Service Set (BSS), periodically broadcasts beacon frames to enableany STAs within wireless range of the AP to establish and maintain acommunication link with the WLAN. In a typical WLAN, only one STA mayuse the wireless medium at any given time, and each STA may beassociated with only one AP at a time.

In many WLANs, data packets are transmitted as orthogonalfrequency-division multiplexing (OFDM) symbols. When a first wirelessdevice transmits an OFDM symbol to a second wireless device, the width(or duration) of the OFDM symbol may increase because of delaysassociated with the transmit (TX) filter in the first wireless device,multipath effects associated with the wireless medium, and delaysassociated with the receive (RX) filter in the second wireless device.

SUMMARY

The systems, methods and devices of this disclosure each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein.

One innovative aspect of the subject matter described in this disclosurecan be implemented in a wireless network to select a guard interval fortransmission of orthogonal frequency-division multiplexing (OFDM)symbols on an uplink (UL) channel and on a downlink (DL) channel. Atransmitting device and a receiving device can estimate channel delayspread (CDS) information for each other's transmit channels, exchangethe estimated CDS information with each other, and select a guardinterval based on the estimated CDS information provided by the otherdevice. A ranging operation may be performed between the transmittingdevice and the receiving device. The transmitting device can estimateCDS information for the DL channel based on frames received from thereceiving device on the DL channel, and the receiving device canestimate CDS information for the UL channel based on frames receivedfrom the transmitting device on the UL channel. The transmitting devicecan send the estimated CDS information for the DL channel to thereceiving device, which can select a guard interval based on theestimated CDS information for the DL channel. Similarly, the receivingdevice can send the estimated CDS information for the UL channel to thetransmitting device, which can select a guard interval based on theestimated CDS information for the UL channel.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented as a method for selecting a guard intervalfor transmission of OFDM symbols on the UL channel. The method caninclude transmitting, to the receiving device on the UL channel, a finetiming measurement (FTM) request frame indicating the CDS capabilitiesof the transmitting device; receiving, from the receiving device on adownlink (DL) channel, a first FTM frame indicating CDS capabilities ofthe receiving device; estimating a CDS value of the DL channel based onthe first FTM frame; transmitting, to the receiving device on the ULchannel, a first acknowledgement (ACK) frame; receiving, from thereceiving device on the DL channel, a second FTM frame indicating anestimated CDS value of the UL channel; and selecting a value for theguard interval based on the estimated CDS value of the UL channel. Themethod also can include transmitting, to the receiving device on the ULchannel, a number of OFDM symbols separated by the selected guardinterval. In some aspects, the method also can include transmitting, tothe receiving device on the UL channel, an FTM feedback frame indicatingthe estimated CDS value of the DL channel.

The FTM request frame can include a vendor-specific information element(VSIE) indicating the CDS capabilities of the transmitting device. Insome aspects, the VSIE of the FTM request frame can include a dedicatedbit indicating whether the transmitting device is capable of estimatingCDS information for the DL channel or whether the transmitting device iscapable of transmitting an FTM feedback frame to the receiving device.The first FTM frame can include a VSIE indicating the CDS capabilitiesof the receiving device. In some aspects, the VSIE of the first FTMframe can include a dedicated bit indicating whether the receivingdevice is capable of estimating CDS information for the UL channel orwhether the receiving device is to report estimated CDS information forthe UL channel to the transmitting device.

In some implementations, the estimated CDS value of the UL channel canbe included in a first of a time of arrival (TOA) field of the secondFTM frame and a time of departure (TOD) field of the second FTM frame,and a time value indicating a difference between the TOD of the firstFTM frame and the TOA of the first ACK frame is included in a second ofthe TOA field of the second FTM frame and the TOD field of the secondFTM frame. In other implementations, the estimated CDS value of the ULchannel can be included in a TOA error field or a TOD error field of thesecond FTM frame.

Another innovative aspect of the subject matter described in thisdisclosure can be implemented as a method for selecting a guard intervalfor transmission of OFDM symbols on the DL channel. The method caninclude receiving, from a transmitting device on an uplink (UL) channel,a fine timing measurement (FTM) request frame indicating a number ofchannel delay spread (CDS) capabilities of the transmitting device;transmitting, to the transmitting device on the DL channel, a first FTMframe indicating CDS capabilities of the receiving device; receiving,from the transmitting device on the UL channel, a first acknowledgement(ACK) frame; estimating a CDS value of the UL channel based on the firstACK frame; transmitting, to the transmitting device on the DL channel, asecond FTM frame indicating the estimated CDS value of the UL channel;receiving, from the transmitting device on the UL channel, an FTMfeedback frame indicating an estimated CDS value of the DL channel; andselecting a value for the guard interval based on the estimated CDSvalue of the DL channel. The method also can include transmitting, tothe transmitting device on the DL channel, a number of OFDM symbolsseparated by the selected guard interval.

Details of one or more implementations of the subject matter describedin this disclosure are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an example wireless system.

FIG. 2 shows a block diagram of an example wireless device.

FIG. 3 shows a diagram depicting an example transmission of OFDM symbolsbetween two wireless devices.

FIG. 4 shows a signal diagram of an example ranging operation.

FIG. 5A shows a signal diagram of another example ranging operation.

FIG. 5B shows a sequence diagram depicting the example ranging operationof FIG. 5A.

FIG. 6A shows an example FTM request frame.

FIG. 6B shows an example FTM action frame.

FIG. 7 shows an example channel delay spread information element (IE).

FIG. 8 shows an illustrative flow chart depicting an example operationfor selecting a guard interval for uplink transmissions.

FIG. 9 shows an illustrative flow chart depicting an example operationfor selecting a guard interval for downlink transmissions.

DETAILED DESCRIPTION

The following description is directed to certain implementations for thepurposes of describing the innovative aspects of this disclosure.However, a person having ordinary skill in the art will readilyrecognize that the teachings herein can be applied in a multitude ofdifferent ways. The described implementations may be implemented in anydevice, system or network that is capable of transmitting and receivingRF signals according to any of the IEEE 16.11 standards, or any of theIEEE 802.11 standards, the Bluetooth® standard, code division multipleaccess (CDMA), frequency division multiple access (FDMA), time divisionmultiple access (TDMA), Global System for Mobile communications (GSM),GSM/General Packet Radio Service (GPRS), Enhanced Data GSM Environment(EDGE), Terrestrial Trunked Radio (TETRA), Wideband-CDMA (W-CDMA),Evolution Data Optimized (EV-DO), 1×EV-DO, EV-DO Rev A, EV-DO Rev B,High Speed Packet Access (HSPA), High Speed Downlink Packet Access(HSDPA), High Speed Uplink Packet Access (HSUPA), Evolved High SpeedPacket Access (HSPA+), Long Term Evolution (LTE), AMPS, or other knownsignals that are used to communicate within a wireless, cellular orinternet of things (IOT) network, such as a system utilizing 3G, 4G or5G, or further implementations thereof, technology.

Increases in the width (or duration) of OFDM symbols caused by filterdelays in a transmitting device, filter delays in a receiving device,and multipath effects of a wireless channel may be referred to aschannel delay spread (CDS). To minimize inter-symbol interference (ISI)resulting from CDS, a guard interval may be provided between OFDMsymbols. The IEEE 802.11a/g/n/ac standards allow two guard intervals(GIs) for OFDM transmissions: a default GI of 0.8 microseconds (μs) anda “short” GI of 0.4 μs. Because CDS values associated with an UL channeland a DL channel may not be readily available, wireless devicestypically select the default GI of 0.8 μs to minimize ISI caused by CDS.Although longer guard intervals may reduce ISI resulting from CDS,longer guard intervals may reduce channel efficiency. Conversely,although shorter guard intervals may increase channel efficiency,shorter guard intervals may result in higher packet error rates.

The IEEE 802.11ax specification addresses outdoor wirelesscommunications, which may have longer channel delay spreads than indoorwireless communications. These longer channel delay spreads may requirelonger guard intervals and longer OFDM symbol lengths (such as comparedwith the IEEE 802.11a/g/n/ac standards). For example, the IEEE 802.11axspecification may allow four guard intervals (GIs) for OFDMtransmissions: a first GI of 3.2 μs, a second GI of 1.6 μs, and the 0.8μs and 0.4 μs GIs defined in the IEEE 802.11a/g/n/ac standards. The CDSvalues associated with UL and DL channels may not be readily available,and therefore wireless devices typically select the longest GI tominimize ISI caused by CDS. Because the longest GI specified in the IEEE802.11ax standards is four times as long as the longest GI specified inthe IEEE 802.11a/g/n/ac standards, it is increasingly important toselect a GI value that minimizes ISI caused by CDS while maximizingchannel efficiency.

Implementations of the subject matter described in this disclosure maybe used to a select a guard interval for transmission of OFDM symbols onan UL channel and on a DL channel. In some aspects, a transmittingdevice and a receiving device can estimate channel delay spread (CDS)information for each other's transmit channels, exchange the estimatedCDS information with each other, and select a guard interval based onthe estimated CDS information exchanged with each other. Thetransmitting device can transmit a number of OFDM symbols separated bythe selected guard interval to the receiving device on the UL channel,and the receiving device can transmit a number of OFDM symbols separatedby the selected guard interval to the transmitting device on the DLchannel.

In some implementations, the CDS capabilities of the transmitting devicecan be provided to the receiving device in an FTM request frame. The FTMrequest frame can include a vendor-specific information element (VSIE)indicating the CDS capabilities of the transmitting device. In someaspects, the VSIE can include one or more dedicated bits indicatingwhether the transmitting device is capable of estimating CDS informationfor the DL channel and whether the transmitting device is capable oftransmitting an FTM feedback frame to the receiving device. In otheraspects, the VSIE can include a dedicated bit either indicating whetherthe transmitting device is capable of estimating CDS information for theDL channel or indicating whether the transmitting device is capable oftransmitting an FTM feedback frame to the receiving device.

The CDS capabilities of the receiving device can be provided to thetransmitting device in an FTM frame. The FTM frame can include a VSIEindicating the CDS capabilities of the receiving device. In someaspects, the VSIE can include one or more dedicated bits indicatingwhether the receiving device is capable of estimating CDS informationfor the UL channel and whether the receiving device is to reportestimated CDS information for the UL channel to the transmitting device.In other aspects, the VSIE can include a dedicated bit either indicatingwhether the receiving device is capable of estimating CDS informationfor the UL channel or indicating whether the receiving device is toreport estimated CDS information for the UL channel to the transmittingdevice.

Particular implementations of the subject matter described in thisdisclosure can be implemented to realize one or more of the followingpotential advantages. Because the transmitting device can estimate theCDS of the DL channel and provide the estimated CDS value to thereceiving device, the receiving device can select a guard interval fortransmitting OFDM symbols on the DL channel based on the estimated CDSof the DL channel. Similarly, because the receiving device can estimatethe CDS of the UL channel and provide the estimated CDS value to thetransmitting device, the transmitting device can select a guard intervalfor transmitting OFDM symbols on the UL channel based on the estimatedCDS of the UL channel. In this manner, aspects of the present disclosuremay allow the transmitting device and the receiving device to selectguard intervals that minimize ISI caused by CDS while maximizing channelefficiency.

FIG. 1 shows a block diagram of an example wireless system 100. Thewireless system 100 is shown to include four wireless stationsSTA1-STA4, a wireless access point (AP) 110, and a wireless local areanetwork (WLAN) 120. The WLAN 120 may be formed by a plurality of Wi-Fiaccess points (APs) that may operate according to the IEEE 802.11 familyof standards (or according to other suitable wireless protocols). Thus,although only one AP 110 is shown in FIG. 1 for simplicity, it is to beunderstood that the WLAN 120 may be formed by any number of accesspoints such as the AP 110. The AP 110 is assigned a unique media accesscontrol (MAC) address that is programmed therein by, for example, themanufacturer of the access point. Similarly, each of the stationsSTA1-STA4 is also assigned a unique MAC address. In someimplementations, the wireless system 100 may correspond to amultiple-input multiple-output (MIMO) wireless network, and may supportsingle-user MIMO (SU-MIMO) and multi-user (MU-MIMO) communications.Further, although the WLAN 120 is depicted in FIG. 1 as aninfrastructure BSS, in some other implementations, the WLAN 120 may bean IBSS, an ad-hoc network, or a peer-to-peer (P2P) network (such asoperating according to the Wi-Fi Direct protocols).

Each of the stations STA1-STA4 may be any suitable Wi-Fi enabledwireless device including, for example, a cell phone, personal digitalassistant (PDA), tablet device, laptop computer, or the like. Each ofthe stations STA1-STA4 also may be referred to as a user equipment (UE),a subscriber station, a mobile unit, a subscriber unit, a wireless unit,a remote unit, a mobile device, a wireless device, a wirelesscommunications device, a remote device, a mobile subscriber station, anaccess terminal, a mobile terminal, a wireless terminal, a remoteterminal, a handset, a user agent, a mobile client, a client, or someother suitable terminology. In some implementations, each of thestations STA1-STA4 may include one or more transceivers, one or moreprocessing resources (such as processors, ASICs, or a combination ofboth), one or more memory resources, and a power source (such as abattery). The memory resources may include a non-transitorycomputer-readable medium (such as one or more nonvolatile memoryelements, such as EPROM, EEPROM, Flash memory, a hard drive, etc.) thatstores instructions for performing operations described below withrespect to FIGS. 5A-5B and FIGS. 8-9.

The AP 110 may be any suitable device that allows one or more wirelessdevices to connect to a network (such as a local area network (LAN),wide area network (WAN), metropolitan area network (MAN), and theInternet) via the AP 110 using Wi-Fi, Bluetooth, or any other suitablewireless communication standards. In some implementations, the AP 110may include one or more transceivers, one or more processing resources(such as processors, ASICs, or a combination of both), one or morememory resources, and a power source. The memory resources may include anon-transitory computer-readable medium (such as one or more nonvolatilememory elements, such as EPROM, EEPROM, Flash memory, a hard drive,etc.) that stores instructions for performing operations described belowwith respect to FIGS. 5A-5B and FIGS. 8-9.

FIG. 2 shows a block diagram of an example wireless device 200. Theexample wireless device 200 may be one implementation of the stationsSTA1-STA4 and the AP 110 of FIG. 1. The wireless device 200 may includea physical-layer device (PHY) 210 including at least a number oftransceivers 211 and a baseband processor 212, may include a MAC 220including at least a number of contention engines 221 and frameformatting circuitry 222, may include a processor 230, may include amemory 240, and may include a number of antennas 250(1)-250(n). Thetransceivers 211 may be coupled to the antennas 250(1)-250(n), eitherdirectly or through an antenna selection circuit (not shown forsimplicity). The transceivers 211 may be used to transmit signals to andreceive signals from other suitable wireless devices, and may be used toscan the surrounding environment to detect and identify nearby accesspoints and other wireless devices (such as within wireless range ofwireless device 200). Although not shown in FIG. 2 for simplicity, thetransceivers 211 may include any number of transmit chains to processand transmit signals to other wireless devices via the antennas250(1)-250(n), and may include any number of receive chains to processsignals received from the antennas 250(1)-250(n). In someimplementations, the wireless device 200 may be configured for MIMOoperations. The MIMO operations may include SU-MIMO operations andMU-MIMO operations.

The baseband processor 212 may be used to process signals received fromthe processor 230 and the memory 240 and to forward the processedsignals to the transceivers 211 for transmission via one or more of theantennas 250(1)-250(n), and may be used to process signals received fromone or more of the antennas 250(1)-250(n) via the transceivers 211 andto forward the processed signals to the processor 230 and the memory240.

The contention engines 221 may contend for access to one or more sharedwireless mediums, and also may store packets for transmission over theone or more shared wireless mediums. In some other implementations, thecontention engines 221 may be separate from the MAC 220. For still otherimplementations, the contention engines 221 may be implemented as one ormore software modules (such as stored in the memory 240 or stored inmemory provided within the MAC 220) containing instructions that, whenexecuted by the processor 230, perform the functions of the contentionengines 221.

The frame formatting circuitry 222 may be used to create and formatframes received from the processor 230 and the memory 240 (such as byadding MAC headers to PDUs provided by the processor 230), and may beused to re-format frames received from the PHY 210 (such as by strippingMAC headers from frames received from the PHY 210).

The memory 240 may include a Wi-Fi database 241 that may store locationdata, configuration information, data rates, MAC addresses, and othersuitable information about (or pertaining to) a number of access points,stations, and other wireless devices. The Wi-Fi database 241 also maystore profile information for a number of wireless devices. The profileinformation for a given wireless device may include informationincluding, for example, the wireless device's service set identification(SSID), channel information, received signal strength indicator (RSSI)values, goodput values, channel state information (CSI), and connectionhistory with wireless device 200.

The memory 240 also may include a non-transitory computer-readablemedium (such as one or more nonvolatile memory elements, such as EPROM,EEPROM, Flash memory, a hard drive, and so on) that may store thefollowing software (SW) modules:

-   -   a ranging SW module 242 to determine RTT values and to estimate        the distance between the wireless device 200 and one or more        other devices, for example, as described below for one or more        operations of FIGS. 5A-5B and FIGS. 8-9;    -   a timestamp SW module 244 to capture or record timestamps of        signals received by the wireless device 200 (such as TOA        information) and to capture or record timestamps of signals        transmitted from the wireless device 200 (such as TOD        information), for example, as described below for one or more        operations of FIGS. 5A-5B and FIGS. 8-9    -   a channel delay spread (CDS) estimation SW module 245 to        estimate the channel delay spread associated with packets        received from other wireless devices and to decode channel delay        spread information provided by one or more other wireless        devices, for example, as described below for one or more        operations of FIGS. 5A-5B and FIGS. 8-9;    -   a frame formation and exchange SW module 246 to create,        transmit, and receive frames to and from other wireless devices,        to embed capabilities of the wireless device 200 (such as a        capability to estimate CDS and a capability to provide feedback        during ranging operations) into frames transmitted to other        wireless devices, and to decode the capabilities of other        wireless devices, for example, as described below for one or        more operations of FIGS. 5A-5B and FIGS. 8-9;    -   a positioning SW module 248 to determine the location of the        wireless device 200 based, at least in part, on the distances        determined by the ranging SW module 242, for example, as        described below for one or more operations of FIGS. 5A-5B and        FIGS. 8-9; and    -   a guard interval (GI) selection SW module 249 to select a guard        interval value that achieves an optimal balance between        maximizing data rates on the wireless medium and minimizing        inter-symbol interference (ISI) resulting from the effects of        channel delay spread, for example, as described below for one or        more operations of FIGS. 5A-5B and FIGS. 8-9.        Each software module includes instructions that, when executed        by the processor 230, cause the wireless device 200 to perform        the corresponding functions. The non-transitory        computer-readable medium of the memory 240 thus includes        instructions for performing all or a portion of the operations        of FIGS. 5A-5B and FIGS. 8-9.

The processor 230 may execute the ranging SW module 242 to determine RTTvalues and to estimate the distance between the wireless device 200 andone or more other devices based on a number of ranging frames exchangedbetween wireless device 200 and the one or more other wireless devices.

The processor 230 may execute the timestamp SW module 244 to capture orrecord timestamps of signals received by the wireless device 200 (suchas TOA information) and to capture or record timestamps of signalstransmitted from the wireless device 200 (such as TOD information). Morespecifically, the timestamp SW module 244 may be executed to capture orrecord TOA information of FTM frames, TOA information of ACK frames, TODinformation of FTM frames, and TOD information of ACK frames.

The processor 230 may execute the CDS estimation SW module 245 toestimate the channel delay spread associated with packets received fromother wireless devices and to decode channel delay spread informationprovided by one or more other wireless devices.

The processor 230 may execute the frame formation and exchange SW module246 to create, transmit, and receive frames to and from other wirelessdevices, to embed capabilities of the wireless device 200 (such as acapability to estimate CDS and a capability to provide feedback duringranging operations) into frames transmitted to other wireless devices,and to decode the capabilities of other wireless devices. The framescreated, transmitted, and received by execution of the frame formationand exchange SW module 246 may be any suitable frames including, forexample, action frames, control frames, management frames, and dataframes. The management frames may include any suitable type of FTMframes (such as FTM request frames, FTM action frames, and FTM feedbackframes), any suitable type of beacon frames, any suitable type of proberequest and probe response frames, any suitable type of associationrequest and association response frames, or/or any suitable type of ACKframes.

The processor 230 may execute the positioning SW module 248 to determinethe location of the wireless device 200 based, at least in part, on thedistances determined by the ranging SW module 242. For example, thepositioning SW module 248 may be executed to determine the relativeposition of the wireless device 200 from the distances between thewireless device 200 and three other devices (such as using knowntrilateration techniques). If the locations of the three other devicesare known, then the actual position of wireless device 200 may bedetermined.

The processor 230 may execute the guard interval (GI) selection SWmodule 249 to select a guard interval value that achieves an optimalbalance between maximizing data rates on the wireless medium andminimizing inter-symbol interference (ISI) resulting from the effects ofchannel delay spread.

FIG. 3 shows a diagram 300 depicting an example transmission of OFDMsymbols between two wireless devices. As shown in FIG. 3, packets aretransmitted as OFDM symbols between a first wireless device (device D1)and a second wireless device (device D2). Device D1 and device D2 mayeach be one implementation of the wireless device 200 of FIG. 2. DeviceD1, which also may be referred to herein as the transmitting device, isshown to include a baseband (BB) processor 212(1), a number of antennas250(1), a transmit chain including at least a transmit filter TX(1), anda receive chain including at least a receive filter RX(1). Device D2,which also may be referred to herein as the receiving device, is shownto include a BB processor 212(2), a number of antennas 250(2), atransmit chain including at least a transmit filter TX(2), and a receivechain including at least a receive filter RX(2). Device D1 may transmitan OFDM symbol 301 to device D2 on an uplink (UL) channel, and device D2may transmit an OFDM symbol 302 to device D1 on a downlink (DL) channel.

When OFDM symbol 301 is transmitted from device D1, multipath effectsand other imperfections in the UL channel may produce signal reflectionsor echoes that result in a channel delayed OFDM symbol 301′ to appear atdevice D2. The increased width of the channel delayed OFDM symbol 301′is denoted as CDS_(DL). Similarly, when OFDM symbol 302 is transmittedfrom device D2, multipath effects and other imperfections in the DLchannel may produce signal reflections or echoes that result in achannel delayed OFDM symbol 302′ to appear at device D1. The increasedwidth of the channel delayed OFDM symbol 302′ is denoted as CDS_(DL).

Device D1 may minimize ISI caused by multipath effects and otherimperfections in the UL channel by selecting a GI value that is greaterthan CDS_(DL), and device D2 may minimize ISI caused by multipatheffects and other imperfections in the DL channel by selecting a GIvalue that is greater than CDS_(DL). For example, device D1 may estimateDL channel information based on packets received from device D2,estimate the CDS for the DL channel based on the estimated channelinformation, and then select a GI value based on the estimated CDS valuefor the DL channel Thereafter, device D1 may transmit OFDM symbols onthe UL channel using the selected GI.

Because device D1 cannot estimate the CDS value for the UL channel,selecting a GI for UL transmissions based on estimated CDS values forthe DL channel assumes that the UL and DL channels have the same CDS,which may not be accurate. In addition, the CDS associated withtransmission of OFDM symbol 301 on the UL channel may depend not only onthe delay characteristics of device D1's transmit filter TX(1) but alsoon the delay characteristics of device D2's receive filter RX(2).Because the transmit and receive filters of Wi-Fi devices are typicallydifferent, the delay characteristics of device D1's transmit filterTX(1) may be different than the delay characteristics of device D2'sreceive filter RX(2). In a similar manner, the CDS associated withtransmission of OFDM symbol 302 on the DL channel may depend not onlythe delay characteristics of device D2's transmit filter TX(2) but alsoon the delay characteristics of device D1's receive filter RX(1).

Further, if OFDM symbol 301 is transmitted as a single spatial streamusing multiple antennas, then device D1 may introduce cyclic shiftdiversity (CSD) between the transmit antennas, for example, to preventbeamforming. The cyclic shift diversity introduced by device D1 mayappear as an additional channel delay to device D2, which may furtherreduce the accuracy of using a CDS estimate of the UL channel as a basisfor selecting a GI for DL transmissions.

In some implementations, wireless devices may use a ranging operation toestimate the CDS of each other's transmit channels and then exchangetheir respective CDS estimates. Each of the wireless devices may use theestimated CDS provided by the other wireless device to accurately selectan optimum GI for data transmissions. In some aspects, a receivingdevice may provide its estimated CDS value for the UL channel to atransmitting device, which may allow the transmitting device to select aGI based on the CDS of the UL channel (such as rather than on the CDS ofthe DL channel). Similarly, the transmitting device may provide itsestimated CDS value for the DL channel to the receiving device, whichmay allow the receiving device to select a GI based on the CDS of the DLchannel (such as rather than on the CDS of the UL channel).

FIG. 4 shows a signal diagram of an example ranging operation 400. Theranging operation 400 may be performed between device D1 and device D2using Fine Timing Measurement (FTM) frames in accordance with the IEEE802.11REVmc standards. Device D1 and device D2 may each be, for example,an access point (such as AP 110 of FIG. 1), a station (such as one ofstations STA1-STA4 of FIG. 1), or other suitable wireless device (suchas wireless device 200 of FIG. 2). For the example of FIG. 4, device D1requests the ranging operation and may be referred to as initiatordevice (or the transmitting device). Device D2 responds to the requestand may be referred to as the responder device (or the receivingdevice).

Device D1 may request or initiate the ranging operation by transmittingan FTM request (FTM_REQ) frame to device D2. The FTM_REQ frame also mayinclude a request for device D2 to capture timestamps (such as TOAinformation of frames received by device D2 and TOD information offrames transmitted from device D2). Device D2 receives the FTM_REQframe, and may acknowledge the requested ranging operation bytransmitting an acknowledgement (ACK) frame to device D1. The ACK framemay indicate whether device D2 is capable of capturing the requestedtimestamps. It is noted that the exchange of the FTM_REQ frame and theACK frame is a handshake process that not only signals an intent toperform a ranging operation but also allows devices D1 and D2 todetermine whether each other supports capturing timestamps.

At time t_(a1), device D2 transmits a first FTM (FTM_1) frame to deviceD1, and may record the TOD of the FTM_1 frame as time t_(a1). Device D1receives the FTM_1 frame at time t_(a2), and may record the TOA of theFTM_1 frame as time t_(a2). Device D1 responds by transmitting a firstacknowledgement (ACK1) frame to device D2 at time t_(a3), and may recordthe TOD of the ACK1 frame as time t_(a3). Device D2 receives the ACK1frame at time t_(a4), and may record the TOA of the ACK1 frame at timet_(a4). At time t_(b1), device D2 transmits to device D1 a second FTM(FTM_2) frame that includes the timestamps captured at times t_(a1) andt_(a4) (such as the TOD of the FTM_1 frame and the TOA of the ACK1frame). Device D1 receives the FTM_2 frame at time t_(b2), and mayrecord its timestamp as time t_(b2).

Upon receiving the FTM_2 frame at time t_(b2), device D1 has timestampvalues for times t_(a1), t_(a2), t_(a3), and t_(a4) that correspond tothe TOD of the FTM_1 frame transmitted from device D2, the TOA of theFTM_1 frame at device D1, the TOD of the ACK1 frame transmitted fromdevice D1, and the TOA of the ACK1 frame at device D2, respectively.Thereafter, device D1 may determine a first RTT value asRTT₁=(t_(a4)−t_(a3))+(t_(a2)−t_(a1)). Because the value of RTT₁ does notinvolve estimating a short interframe space (SIFS) duration for eitherdevice D1 or device D2, the value of RTT₁ does not involve errorsresulting from uncertainties of SIFS durations. Thereafter, device D1may determine the distance (d) between device D1 and device D2 using theexpression d=c*RTT/2, where c is the speed of light.

As depicted in FIG. 4, devices D1 and D2 are shown to exchange anadditional pair of FTM and ACK frames from which an additional RTT valuemay be determined. Specifically, at time t_(b3), device D1 may transmita second acknowledgement (ACK2) frame to device D2 (such as toacknowledge reception of the FTM_2 frame). Device D2 receives the ACK2frame at time t_(b4), and may record the TOA of the ACK2 frame as timet_(b4). At time t_(c1), device D2 transmits to device D1 a third FTM(FTM_3) frame that includes the timestamps captured at times t_(b1) andt_(b4) (such as the TOD of the FTM_2 frame and the TOA of the ACK2frame). Device D1 receives the FTM_3 frame at time t_(c2), and mayrecord its timestamp as time t_(c2). Device D1 may respond bytransmitting a third FTM acknowledgement (ACK3) frame to device D2 attime t_(c3).

Upon receiving the FTM_3 frame at time t_(c2), device D1 has timestampvalues for times t_(b1), t_(b2), t_(b3), and t_(b4) that correspond tothe TOD of the FTM_2 frame transmitted from device D2, the TOA of theFTM_2 frame at device D1, the TOD of the ACK2 frame transmitted fromdevice D1, and the TOA of the ACK2 frame at device D2, respectively.Thereafter, device D1 may determine a second RTT value asRTT₂=(t_(b4)−t_(b3))+(t_(b2)−t_(b1)). This process may continue for anynumber of subsequent FTM and ACK frame exchanges between devices D1 andD2, for example, where device D2 embeds the timestamps of a given FTMand ACK frame exchange into a subsequent FTM frame transmitted to deviceD1. By determining multiple RTT values between devices D1 and D2,ranging accuracy may be improved by using the multiple RTT values toaverage out noise and to eliminate outlier RTT values (such as RTTvalues that are more than a given deviation from an average RTT valuebetween devices D1 and D2).

The example ranging operation 400 of FIG. 4 may be modified to allowdevice D1 to estimate the CDS of the DL channel (denoted herein asCDS_(DL)) and provide the estimated CDS_(DL) to device D2, and also mayallow device D2 to estimate the CDS of the UL channel (denoted herein asCDS_(DL)) and provide the estimated CDS_(DL) to device D1. In thismanner, device D1 may select an optimum GI value for UL transmissionsthat is based on CDS estimates of the UL channel, and device D2 mayselect an optimum GI value for DL transmissions that is based on CDSestimates of the DL channel.

FIG. 5A shows a signal diagram of another example ranging operation 500,and FIG. 5B shows a sequence diagram 510 depicting the example rangingoperation 500 of FIG. 5A. The example ranging operation 500 may allowdevices D1 and D2 to exchange CDS_(DL) and CDS_(DL) information, inaccordance with aspects of the present disclosure. Devices D1 and D2 mayeach be, for example, an access point (such as the AP 110 of FIG. 1), astation (such as one of the stations STA1-STA4 of FIG. 1), or anothersuitable wireless device (such as the wireless device 200 of FIG. 2).Referring also to FIG. 3, device D1 transmits signals to device D2 onthe UL channel, and device D2 transmits signals to device D1 on the DLchannel Device D1 may be the transmitting device, and device D2 may bethe receiving device.

At time t₁, device D1 may request the ranging operation 500 bytransmitting, to device D2, an FTM_REQ frame that includes channel delayspread (CDS) capabilities of device D1 (511). The CDS capabilities mayindicate whether device D1 is capable of estimating CDS values based onpackets received from device D2 on the DL channel, and may indicatewhether device D1 is capable of transmitting (to device D2) a feedbackframe that includes the estimated CDS values (and other information suchas, for example, RTT information). The FTM_REQ frame also may requestdevice D2 to capture or record timestamps, for example, in a mannersimilar to that described with respect to FIG. 4.

The CDS capabilities of device D1 may be included in or transmitted withthe FTM_REQ frame in any suitable manner. In some implementations, theFTM_REQ frame may include a vendor-specific information element (VSIE)that indicates the CDS capabilities of device D1. The VSIE may beembedded within or appended to the FTM_REQ frame in any suitable manner.In some aspects, the VSIE may include a first dedicated bit to indicatewhether device D1 is capable of estimating CDS information based on FTMframes received from device D2, and may include a second dedicated bitto indicate whether device D1 is capable of transmitting theaforementioned feedback frame (such as that includes estimated CDSvalues and RTT information) to device D2. For example, the firstdedicated bit may be set to a value of “1” to indicate that device D1 iscapable of estimating CDS information, or may be set to a value of “0”to indicate that device D1 is not capable of estimating CDS information.Similarly, the second dedicated bit may be set to a value of “1” toindicate that device D1 is capable of transmitting feedback frames, ormay be set to a value of “0” to indicate that device D1 is not capableof transmitting feedback frames.

In some other aspects, the VSIE may include one dedicated bit either toindicate whether device D1 is capable of estimating CDS information forthe DL channel based on FTM frames received from device D2 or toindicate whether device D1 is capable of transmitting the FTM_FB frameto device D2.

In some implementations, one of the reserved bits in an informationelement (IE) of the FTM_REQ frame may be used to indicate whether deviceD1 is capable of estimating CDS information based on FTM frames receivedfrom device D2 (such as where a value of “1” may indicate that device D1is capable of estimating CDS information, and a value of “0” mayindicate that device D1 is not capable of estimating CDS information),and another one of the reserved bits in the IE of the FTM_REQ frame maybe used to indicate whether device D1 is capable of transmittingfeedback frames (such as where a value of “1” may indicate that deviceD1 is capable of transmitting feedback frames, and a value of “0” mayindicate that device D1 is not capable of transmitting feedback frames).

In some other implementations, device D1 and device D2 may exchange CDScapability information prior to the example ranging operation 500. Forexample, if device D1 (or device D2) is an access point or a group owner(GO), then device D1 (or device D2) may embed or append a CDScapabilities IE in beacon frames, for example, so that other wirelessdevices within wireless range are aware of the CDS capabilities ofdevice D1 (or device D2). For another example, if device D1 (or deviceD2) is a station, then device D1 (or device D2) may embed or append aCDS capability IE in a probe request or an association request (or anyother suitable frame), for example, to announce its CDS capabilities.

At time t₂, device D2 receives the FTM_REQ frame, and may decode the CDScapabilities of device D1 (512). Device D2 may acknowledge the requestedranging operation 500 by transmitting an ACK frame to device D1 at timet₃ (513). In some aspects, the ACK frame may indicate whether device D2is capable of estimating CDS information based on packets received fromdevice D1. Device D1 receives the ACK frame at time t₄, and may preparefor one or more FTM frame exchanges (514).

At time t_(a1), device D2 transmits, to device D1, an FTM_1 frame thatincludes its CDS capabilities, and may record the TOD of the FTM_1 frameas time t_(a1) (515). The CDS capabilities of device D2 may be includedin or transmitted with the FTM_1 frame in any suitable manner. In someaspects, the FTM_1 frame may include a VSIE that indicates the CDScapabilities of device D1. The VSIE, which may be embedded within orappended to the FTM_1 frame in any suitable manner, may include adedicated bit to indicate whether device D2 is capable of (and intendsto) embed estimated CDS information in subsequent FTM frames. Forexample, device D2 may set the dedicated bit to a value of “1” toindicate that device D2 will embed CDS information in subsequent FTMframes, or may set the dedicated bit to a value of “0” to indicate thatdevice D2 will not embed CDS information in subsequent FTM frames. Insome other aspects, device D2 may not embed the VSIE into the FTM_1frame, for example, to inform device D1 that device D2 will not embedCDS information in subsequent FTM frames (thereby reducing the size ofthe FTM_1 frame).

At time t_(a2), device D1 receives the FTM_1 frame, may decode the CDScapabilities of device D2 (if provided in the FTM_1 frame), and mayrecord the TOA of the FTM_1 frame as time t_(a2) (516). Device D1 mayobtain DL channel information based on the FTM_1 frame received fromdevice D2, and then use the channel information to estimate a first CDSvalue for the DL channel (CDS_(DL1)) (517). At time to, device D1 maytransmit an ACK1 frame to device D2 and record the TOD of the ACK1 frameas time t_(a3) (518).

Device D2 receives the ACK1 frame at time t_(a4), and may record the TOAof the ACK1 frame as time t_(a4) (519). Device D2 may obtain UL channelinformation based on the ACK1 frame received from device D1, and thenuse the channel information to estimate a first CDS value for the ULchannel (CDS_(UL1)) (520).

Then, at time t_(b1), device D2 may embed timing information, theestimated value of CDS_(UL1), or both into an FTM_2 frame, transmit theFTM_2 frame to device D1, and record the TOD of the FTM_2 frame as timet_(b1) (521). In some aspects, the timing information may indicate adifference between times t_(a4) and t_(a1) (such ast_(diff1)=t_(a4)−t_(a1)), for example, as depicted in FIG. 5A. In someother aspects, the timing information may include timestamps t_(a1) andt_(a4) that correspond to the TOD of the FTM_1 frame transmitted fromdevice D2 and the TOA of the ACK1 frame received at device D2,respectively.

At time t_(b2), device D1 receives the FTM_2 frame, decodes the embeddedtiming information and the value of CDS_(UL1), and records the TOA ofthe FTM_2 frame as time t_(b2) (522). Device D1 may obtain DL channelinformation based on the FTM_2 frame received from device D2, and thenuse the channel information to estimate a second CDS value for the DLchannel (CDS_(DL2)) (523). At time t_(b3), device D1 may transmit anACK2 frame to device D2 and record the TOD of the ACK2 frame as timet_(b3) (524).

Upon receiving the FTM_2 frame at time t_(b2), device D1 has timinginformation from which a first RTT value between device D1 and device D2may be determined. In some aspects, device D1 may determine the firstRTT value, and may determine the distance between devices D1 and D2based on the first RTT value (525). For the example depicted in FIG. 5A,device D1 has timestamp values for times t_(a2) and t_(a3) thatcorrespond to the TOA of the FTM_1 frame at device D1 and the TOD of theACK1 frame transmitted from device D1, and has a difference valuet_(diff1) indicative of the difference in time between the TOD of theFTM_1 frame transmitted from device D2 (t_(a1)) and the TOA of the ACK1frame at device D2 (t_(a4)). Thereafter, device D1 may determine thefirst RTT value as RTT₁=t_(diff)−(t_(a3)−t_(a2)). In some otherimplementations in which device D2 embeds timestamp values t_(a1) andt_(a4) into the FTM_2 frame, device D1 may determine the first RTT valueas RTT₁=(t_(a4)−t_(a3))+(t_(a2)−t_(a1)).

In addition, upon receiving the FTM_2 frame at time t_(b2), device D1has CDS information for the UL channel (such as CDS_(UL1)). In someimplementations, device D1 may use the value of CDS_(UL1) to select a GIvalue that achieves an optimal balance between minimizing ISI caused bychannel delay spread and maximizing the efficiency of the UL channelThus, for example, if the value of CDS_(UL1) is between 3.2 μs and 1.6μs, then device D1 may select the 3.2 μs GI specified by the IEEE802.11ax standards.

For another example, if the value of CDS_(UL1) is between 1.6 μs and 0.8μs, then device D1 may select the 1.6 μs GI specified by the IEEE802.11ax standards, and use the 1.6 μs GI for subsequent transmissionsof OFDM symbols. In this example, the 1.6 μs GI is greater than the CDSof the UL channel (thereby minimizing ISI caused by CDS), and isone-half the duration of the longest GI of 3.2 μs. As a result, deviceD1 may be able to achieve higher data rates for OFDM symboltransmissions on the UL channel, for example, as compared to OFDM symboltransmissions using the longest GI of 3.2 μs.

For another example, if the value of CDS_(UL1) is between 0.8 μs and 0.4μs, then device D1 may select the 0.8 μs GI specified by the IEEE802.11ax standards, and use the 0.8 μs GI for subsequent transmissionsof OFDM symbols. In this example, the 0.8 μs GI is greater than the CDSof the UL channel (thereby minimizing ISI caused by CDS), and isone-fourth the duration of the longest GI of 3.2 μs. As a result, deviceD1 may be able to achieve higher data rates for OFDM symboltransmissions on the UL channel, for example, as compared to OFDM symboltransmissions using the 1.6 μs GI.

For yet another example, if the value of CDS_(UL1) is less than 0.4 μs,then device D1 may select the 0.4 μs GI specified by the IEEE 802.11axstandards, and use the 0.4 μs GI for subsequent transmissions of OFDMsymbols. In this example, the 0.4 μs GI is greater than the CDS of theUL channel (thereby minimizing ISI caused by CDS), and is one-eighth theduration of the longest GI of 3.2 μs. As a result, device D1 may be ableto achieve higher data rates for OFDM symbol transmissions on the ULchannel, for example, as compared to OFDM symbol transmissions using the0.8 μs GI.

Because device D1 may select the GI value based on the estimated CDS ofthe UL channel (such as rather than assuming that the CDS of the ULchannel is the same as the CDS of the DL channel), the GI selected bydevice D1 may be more accurately based on the actual channel conditionsof the UL channel. As a result, device D1 may be able to maximize ULchannel efficiency (by selecting a minimum GI value) while minimizingpacket error rates (by selecting a GI value that is greater than the CDSof the UL channel).

Device D2 receives the ACK2 frame at time t_(b4), and may record the TOAof the ACK2 frame as time t_(b4) (526). Device D2 may obtain UL channelinformation based on the ACK2 frame received from device D1, and thenuse the channel information to estimate a second CDS value for the ULchannel (CDS_(UL2)) (527).

This process may continue for any number of subsequent FTM and ACK frameexchanges, where each subsequent FTM and ACK frame exchange may allowdevice D2 to estimate another CDS value for the UL channel and may allowdevice D1 to estimate another CDS value for the DL channel and todetermine another RTT value. For example, as depicted in FIG. 5A,devices D1 and D2 exchange a third set of FTM and ACK frames during theranging operation 500.

More specifically, at time t_(c1), device D2 may embed timinginformation, the value of CDS_(UL2), or both into an FTM_3 frame,transmit the FTM_3 frame to device D1, and record the TOD of the FTM_3frame as time t_(c1). In some aspects, the timing information mayinclude a difference value t_(diff2)=t_(b4)−t_(b1), while in some otheraspects, the timing information may include timestamps t_(b1) andt_(b4).

At time t_(c2), device D1 receives the FTM_3 frame, decodes the embeddedtiming information and the value of CDS_(UL2), and records the TOA ofthe FTM_3 frame as time t_(c2). Device D1 may obtain DL channelinformation based on the FTM_3 frame received from device D2, and thenuse the channel information to estimate a third CDS value for the DLchannel (CDS_(DL3)). At time t_(c3), device D1 may transmit an ACK3frame to device D2 and record the TOD of the ACK3 frame as time t_(c3).

Upon receiving the FTM_3 frame at time t_(c2), device D1 has timinginformation from which a second RTT value between device D1 and deviceD2 may be determined. In some aspects, device D1 may determine thesecond RTT value, and may determine the distance between devices D1 andD2 based on the second RTT value. For the example depicted in FIG. 5A,device D1 has timestamp values for times t_(b2) and t_(b3) thatcorrespond to the TOA of the FTM_2 frame at device D1 and the TOD of theACK2 frame transmitted from device D1, and has a difference valuet_(diff2) indicative of the difference in time between the TOD of theFTM_2 frame transmitted from device D2 (t_(b1)) and the TOA of the ACK2frame at device D2 (t_(b4)). Thereafter, device D1 may determine thesecond RTT value as RTT₂=t_(diff2)−(t_(b3)−t_(b2)). In some otherimplementations in which device D2 embeds timestamp values t_(b1) andt_(b4) into the FTM_3 frame, device D1 may determine the second RTTvalue as RTT₁=(t_(b4)−t_(b3))+(t_(b2)−t_(b1)).

In addition, upon receiving the FTM_3 frame at time t_(c2), device D1has additional CDS information for the UL channel (such as CDS_(UL2)).In some implementations, device D1 may determine an average CDS valuefor the UL channel based on CDS_(UL1) and CDS_(UL2), which in turn mayprovide a more accurate estimate the channel delay spread of the ULchannel.

Thereafter, device D1 may transmit, to device D2, an FTM feedback frame(FTM_FB) that includes RTT information and CDS information for the DLchannel, at time t_(d1) (528). In some aspects, device D1 may determinean average DL CDS value (CDS_(DL) _(_) _(ave)) based on one or more ofthe previously estimated DL CDS values (such as CDS_(DL1), CDS_(DL2),and CDS_(DL3)), may determine an average RTT value (RTT_(ave)) based onone or more of the previously estimated RTT values (such as RTT₁ andRTT₂), and may embed values for CDS_(DL) _(_) _(ave) and RTT_(ave) intothe FTM_FB frame. In some other aspects, instead of embedding the valueRTT_(ave) into the FTM_FB frame, device D1 may embed an average of thedifferences between times t_(a3)−t_(a2), times t_(b3)−t_(b2), timest_(c3)−t_(c2), or any combination thereof into the FTM_FB frame, oralternatively may embed a selected one of the RTT values (such as one ofRTT₁, RTT₂, and so on).

At time t_(d2), device D2 may receive the FTM_FB frame, and decode theembedded RTT information and the CDS information for the DL channel(529). At time t_(d3), device D2 may transmit an ACK4 frame to deviceD1. Device D1 may receive the ACK4 frame at time 414.

Upon receiving the FTM_FB frame at time t_(d2), device D2 has CDSinformation for the DL channel (such as CDS_(DL) _(_) _(ave)). In someimplementations, device D2 may use the value of CDS_(DL) _(_) _(ave) toselect a GI value that achieves an optimal balance between minimizingISI caused by channel delay spread and maximizing the efficiency of theDL channel. In some aspects, device D2 may select a GI value based onthe value of CDS_(DL) _(_) _(ave) in a manner similar to that describedwith respect to device D1 selecting a GI based on CDS information forthe UL channel.

It is noted that because device D2 may select the GI value based on theestimated CDS of the DL channel (such as rather than assuming that theCDS of the DL channel is the same as the CDS of the UL channel), the GIselected by device D2 may be more accurately based on the actual channelconditions of the DL channel. As a result, device D2 may be able tomaximize DL channel efficiency (by selecting a minimum GI value) whileminimizing packet error rates (by selecting a GI value that is greaterthan the CDS of the DL channel).

FIG. 6A shows an example FTM_REQ frame 600. In some implementations, theFTM_REQ frame 600 may be used in the example ranging operation 500 ofFIG. 5A. The FTM_REQ frame 600 may include a category field 601, apublic action field 602, a trigger field 603, an optional location civicinformation (LCI) measurement request field 604, an optional locationcivic measurement request field 605, an optional FTM parameters field606, and a CDS capabilities VSIE 607. The fields 601-606 of the FTM_REQframe 600 are well-known, and therefore are not discussed in detailherein.

The CDS capabilities VSIE 607 may store CDS capabilities of theinitiator device (such as device D1), for example, as described withrespect to FIG. 5A. In some other implementations, the CDS capabilitiesVSIE 607 may be an information element (IE).

FIG. 6B shows an example FTM frame 610. In some implementations, the FTMframe 610 may be used as one or more of the FTM_1 frame, the FTM_2frame, the FTM_3 frame, and the FTM_FB frame in the example rangingoperation 500 of FIG. 5A. The FTM frame 610 may include a category field611, a public action field 612, a dialog token field 613, a follow updialog token field 614, a TOD field 615, a TOA field 616, a TOD errorfield 617, a TOA error field 618, an optional LCI report field 619, anoptional location civic report field 620, an optional FTM parametersfield 621, and a CDS capabilities VSIE 622. The fields 611-621 of theFTM frame 610 are well-known, and therefore are not discussed in detailherein.

In some implementations, the TOD field 615 may include 6 bytes, and theTOA field 616 may include 6 bytes (although in some otherimplementations, other field lengths may be used). The TOD field 615 istypically used by the responder device to report a TOD value (such ast_(a1)) to the initiator device, and the TOA field 616 is typically usedby the responder device to report a TOA value (such as t_(a4)) to theinitiator device. In some implementations, one of the TOD field 615 andTOA field 616 may be used to report the difference time value(t_(diff)=t_(a4)−t_(a1)) to the initiator device, and the other of theTOD field 615 and TOA field 616 may be used to report the UL CDSinformation (such as CDS_(UL)) to the initiator device. In some aspects,the 48 bits of the TOD field 615 may be used to store a value fort_(diff), and the 48 bits of the TOA field 616 may be used to store avalue for CDS_(UL). In some other aspects, the 48 bits of the TOD field615 may be used to store a value for CDS_(UL), and the 48 bits of theTOA field 616 may be used to store a value for t_(diff). In some otherimplementations, the CDS values may be stored in the TOD error field 617or in the TOA error field 618.

The CDS capabilities VSIE 622 may store CDS capabilities of theresponder device (such as device D2), for example, as described withrespect to FIG. 5A. In some other implementations, the CDS capabilitiesVSIE 622 may be an information element (IE).

FIG. 7 shows a CDS capabilities VSIE 700. The CDS capabilities VSIE 700may include an Element ID field 701, a Length field 702, and a CDSinformation field 703. In some implementations, the Element ID field 701may include one byte, the Length field 702 may include one byte, and theCDS information field 703 may include one byte (although in some otherimplementations, other field lengths may be used). The Element ID field701 may store an element ID value indicating that VSIE 700 contains CDScapabilities for a device. The Length field 702 may store a valueindicating a length (in bytes) of the CDS information field 703.

In some aspects, the 8 bits in the CDS information field 703 may storeCDS values up to 2550 nanoseconds (ns) with a resolution of 10 ns. Inthis manner, the four guard intervals specified in the IEEE 802.11axstandards may be expressed as 400 ns, 800 ns, 1600 ns, and 3200 ns,respectively. If the channel delay spread is longer than 2550 ns, thenthe 8 bits in the CDS information field 703 may be set to indicate avalue of 2550 ns. In response thereto, a receiving may select a GI valueof 3200 (such as 3.2 μs).

In some other implementations, the CDS information field 703 may include2 bits (such as rather than 8 bits) that may indicate up to four rangesof CDS values. For example, in some aspects, a value of “00” mayindicate that the CDS value is less than 0.4 μs, a value of “01” mayindicate that the CDS value is between 0.4 μs and 0.8 μs, a value of“10” may indicate that the CDS value is between 0.8 μs and 1.6 μs, and avalue of “11” may indicate that the CDS value is between 1.6 μs and 3.2μs or greater than 3.2 μs.

FIG. 8 shows an illustrative flow chart depicting an example operation800 for selecting a guard interval for uplink transmissions. In someimplementations, the example operation 800 may include the transmissionof OFDM symbols from a transmitting device to a receiving device on anuplink (UL) channel. The transmitting device and the receiving devicemay be, for example, an access point (such as the AP 110 of FIG. 1), astation (such as one of the stations STA1-STA4 of FIG. 1), or anysuitable wireless device (such as the wireless device 200 of FIG. 2).

The transmitting device transmits, to the receiving device on the ULchannel, an FTM request frame indicating channel delay spread (CDS)capabilities of the transmitting device (802). The CDS capabilities mayindicate whether the transmitting device is capable of estimating CDSvalues based on packets received from the receiving device on the DLchannel and may indicate whether the transmitting device is capable oftransmitting (to the receiving device) a feedback frame that includesthe estimated CDS values and other information such as, for example, RTTinformation.

The transmitting device receives, from the receiving device on adownlink (DL) channel, a first FTM frame indicating CDS capabilities ofthe receiving device (804). The CDS capabilities of the receiving devicemay indicate whether the receiving device is capable of (and intends to)embed estimated CDS information in subsequent FTM frames.

The transmitting device estimates a CDS value of the DL channel based onreception of the first FTM frame (806). For example, the transmittingdevice may obtain DL channel information based on the FTM_1 framereceived from the receiving device, and then use the channel informationto estimate a first CDS value for the DL channel (CDS_(DL1)).

The transmitting device transmits, to the receiving device on the ULchannel, a first acknowledgement (ACK) frame (808), and then receives,from the receiving device on the DL channel, a second FTM frameindicating an estimated CDS value of the UL channel (810). Thetransmitting device may then select a value for the guard interval basedon the estimated CDS value of the UL channel (812). In someimplementations, because the transmitting device may select the GI valuebased on the estimated CDS of the UL channel (such as rather thanassuming that the CDS of the UL channel is the same as the CDS of the DLchannel), the GI selected by the transmitting device may be moreaccurately based on the actual channel conditions of the UL channel.

The transmitting device transmits, to the receiving device on the ULchannel, an FTM feedback frame indicating the estimated CDS value of theDL channel (814). The FTM feedback frame also may include RTTinformation determined by the transmitting device.

Thereafter, the transmitting device may transmit, to the receivingdevice on the UL channel, a number of OFDM symbols separated by theselected guard interval (816). In this manner, the transmitting devicemay minimize ISI caused by CDS while maximizing channel efficiency, forexample, by selecting the shortest GI value that still exceeds the CDSvalue of the UL channel.

FIG. 9 shows an illustrative flow chart depicting an example operation900 for selecting a guard interval for downlink transmission. In someimplementations, the example operation 900 may include the transmissionof OFDM symbols from the receiving device to the transmitting device ona downlink (DL) channel. The transmitting device and the receivingdevice may be, for example, an access point (such as the AP 110 of FIG.1), a station (such as one of the stations STA1-STA4 of FIG. 1), or anysuitable wireless device (such as the wireless device 200 of FIG. 2).

The receiving device receives, from the transmitting device on an ULchannel, an FTM request frame indicating a number of channel delayspread (CDS) capabilities of the transmitting device (902). The CDScapabilities may indicate whether the transmitting device is capable ofestimating CDS values based on packets received from the receivingdevice on the DL channel and may indicate whether the transmittingdevice is capable of transmitting (to the receiving device) a feedbackframe that includes the estimated CDS values and other information suchas, for example, RTT information.

The receiving device transmits, to the transmitting device on the DLchannel, a first FTM frame indicating CDS capabilities of the receivingdevice (904). The CDS capabilities of the receiving device may indicatewhether the receiving device is capable of (and intends to) embedestimated CDS information in subsequent FTM frames.

The receiving device receives, from the transmitting device on the ULchannel, a first acknowledgement (ACK) frame (906), and may thenestimate a CDS value of the UL channel based on reception of the firstACK frame (908). For example, the receiving device may obtain UL channelinformation based on the ACK1 frame received from the receiving device,and then use the channel information to estimate a first CDS value forthe UL channel (CDS_(UL1)).

The receiving device transmits, to the transmitting device on the DLchannel, a second FTM frame indicating the estimated CDS value of the ULchannel (910). The second FTM frame also may include timing informationfrom which the transmitting device may determine one or more RTT values.The receiving device then receives, from the transmitting device on theUL channel, an FTM feedback frame indicating an estimated CDS value ofthe DL channel (912).

In response thereto, the receiving device selects a value for the guardinterval based on the estimated CDS value of the DL channel (914). Insome implementations, because the receiving device may select the GIvalue based on the estimated CDS of the DL channel (such as rather thanassuming that the CDS of the DL channel is the same as the CDS of the ULchannel), the GI selected by the receiving device may be more accuratelybased on the actual channel conditions of the DL channel. As a result,the receiving device may be able to maximize DL channel efficiency whileminimizing packet error rates.

Thereafter, the receiving device may transmit, to the transmittingdevice on the DL channel, a number of OFDM symbols separated by theselected guard interval (916). In this manner, the receiving device mayminimize ISI caused by CDS while maximizing channel efficiency, forexample, by selecting the shortest GI value that still exceeds the CDSvalue of the DL channel.

As used herein, a phrase referring to “at least one of” a list of itemsrefers to any combination of those items, including single members. Asan example, “at least one of: a, b, or c” is intended to cover: a, b, c,a-b, a-c, b-c, and a-b-c.

The various illustrative logics, logical blocks, modules, circuits andalgorithm processes described in connection with the implementationsdisclosed herein may be implemented as electronic hardware, computersoftware, or combinations of both. The interchangeability of hardwareand software has been described generally, in terms of functionality,and illustrated in the various illustrative components, blocks, modules,circuits and processes described throughout. Whether such functionalityis implemented in hardware or software depends upon the particularapplication and design constraints imposed on the overall system.

The hardware and data processing apparatus used to implement the variousillustrative logics, logical blocks, modules and circuits described inconnection with the aspects disclosed herein may be implemented orperformed with a general purpose single- or multi-chip processor, adigital signal processor (DSP), an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA) or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. A general purpose processor may be amicroprocessor, or, any conventional processor, controller,microcontroller, or state machine. A processor also may be implementedas a combination of computing devices (such as a combination of a DSPand a microprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration). In some implementations, particular processes andmethods may be performed by circuitry that is specific to a givenfunction.

In one or more aspects, the functions described may be implemented inhardware, digital electronic circuitry, computer software, firmware,including the structures disclosed in this specification and theirstructural equivalents thereof, or in any combination thereof.Implementations of the subject matter described in this specificationalso can be implemented as one or more computer programs, i.e., one ormore modules of computer program instructions, encoded on a computerstorage media for execution by, or to control the operation of, dataprocessing apparatus.

If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. The processes of a method or algorithmdisclosed herein may be implemented in a processor-executable softwaremodule which may reside on a computer-readable medium. Computer-readablemedia includes both computer storage media and communication mediaincluding any medium that can be enabled to transfer a computer programfrom one place to another. A storage media may be any available mediathat may be accessed by a computer. By way of example, and notlimitation, such computer-readable media may include RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that may be used to storedesired program code in the form of instructions or data structures andthat may be accessed by a computer. Also, any connection can be properlytermed a computer-readable medium. Disk and disc, as used herein,includes compact disc (CD), laser disc, optical disc, digital versatiledisc (DVD), floppy disk, and blu-ray disc where disks usually reproducedata magnetically, while discs reproduce data optically with lasers.Combinations of the above should also be included within the scope ofcomputer-readable media. Additionally, the operations of a method oralgorithm may reside as one or any combination or set of codes andinstructions on a machine readable medium and computer-readable medium,which may be incorporated into a computer program product.

Various modifications to the implementations described in thisdisclosure may be readily apparent to those skilled in the art, and thegeneric principles defined herein may be applied to otherimplementations without departing from the spirit or scope of thisdisclosure. Thus, the claims are not intended to be limited to theimplementations shown herein, but are to be accorded the widest scopeconsistent with this disclosure, the principles and the novel featuresdisclosed herein.

What is claimed is:
 1. A method of selecting a guard interval fortransmission of orthogonal frequency-division multiplexing (OFDM)symbols on an uplink (UL) channel, comprising: transmitting, to areceiving device on the UL channel, a fine timing measurement (FTM)request frame indicating channel delay spread (CDS) capabilities of atransmitting device; receiving, from the receiving device on a downlink(DL) channel, a first FTM frame indicating CDS capabilities of thereceiving device; estimating a CDS value of the DL channel based on thefirst FTM frame; transmitting, to the receiving device on the ULchannel, a first acknowledgement (ACK) frame; receiving, from thereceiving device on the DL channel, a second FTM frame indicating anestimated CDS value of the UL channel; and selecting a value for theguard interval based on the estimated CDS value of the UL channel. 2.The method of claim 1, wherein the FTM request frame includes avendor-specific information element (VSIE) indicating the CDScapabilities of the transmitting device.
 3. The method of claim 2,wherein the VSIE includes a dedicated bit indicating whether thetransmitting device is capable of estimating CDS information for the DLchannel or whether the transmitting device is capable of transmitting anFTM feedback frame to the receiving device.
 4. The method of claim 1,wherein the first FTM frame includes a vendor-specific informationelement (VSIE) indicating the CDS capabilities of the receiving device.5. The method of claim 4, wherein the VSIE includes a dedicated bitindicating whether the receiving device is capable of estimating CDSinformation for the UL channel or whether the receiving device is toreport estimated CDS information for the UL channel to the transmittingdevice.
 6. The method of claim 1, wherein the estimated CDS value of theUL channel is included in a first of a time of arrival (TOA) field ofthe second FTM frame and a time of departure (TOD) field of the secondFTM frame, and a time value indicating a difference between the TOD ofthe first FTM frame and the TOA of the first ACK frame is included in asecond of the TOA field of the second FTM frame and the TOD field of thesecond FTM frame.
 7. The method of claim 1, wherein the estimated CDSvalue of the UL channel is included in a time of arrival (TOA) errorfield or a time of departure (TOD) error field of the second FTM frame.8. The method of claim 1, further comprising: transmitting, to thereceiving device on the UL channel, an FTM feedback frame indicating theestimated CDS value of the DL channel.
 9. The method of claim 1, furthercomprising: transmitting, to the receiving device on the UL channel, anumber of OFDM symbols separated by the selected guard interval.
 10. Anapparatus for selecting a guard interval for transmission of orthogonalfrequency-division multiplexing (OFDM) symbols on an uplink (UL)channel, comprising: one or more processors; and a memory comprisinginstructions that, when executed by the one or more processors, causesthe apparatus to: transmit, to a receiving device on the UL channel, afine timing measurement (FTM) request frame indicating channel delayspread (CDS) capabilities of the apparatus; receive, from the receivingdevice on a downlink (DL) channel, a first FTM frame indicating CDScapabilities of the receiving device; estimate a CDS value of the DLchannel based on the first FTM frame; transmit, to the receiving deviceon the UL channel, a first acknowledgement (ACK) frame; receive, fromthe receiving device on the DL channel, a second FTM frame indicating anestimated CDS value of the UL channel; and select a value for the guardinterval based on the estimated CDS value of the UL channel.
 11. Theapparatus of claim 10, wherein the FTM request frame includes avendor-specific information element (VSIE) indicating the CDScapabilities of the apparatus.
 12. The apparatus of claim 10, whereinthe first FTM frame includes a vendor-specific information element(VSIE) indicating the CDS capabilities of the receiving device.
 13. Theapparatus of claim 12, wherein the VSIE includes a dedicated bitindicating whether the receiving device is capable of estimating CDSinformation for the UL channel or whether the receiving device is toreport estimated CDS information for the UL channel to the apparatus.14. The apparatus of claim 10, wherein the estimated CDS value of the ULchannel is included in a first of a time of arrival (TOA) field of thesecond FTM frame and a time of departure (TOD) field of the second FTMframe, and a time value indicating a difference between the TOD of thefirst FTM frame and the TOA of the first ACK frame is included in asecond of the TOA field of the second FTM frame and the TOD field of thesecond FTM frame.
 15. The apparatus of claim 10, wherein the estimatedCDS value of the UL channel is included in a time of arrival (TOA) errorfield or a time of departure (TOD) error field of the second FTM frame.16. The apparatus of claim 10, wherein execution of the instructionscauses the apparatus to further: transmit, to the receiving device onthe UL channel, a number of OFDM symbols separated by the selected guardinterval.
 17. A method of selecting a guard interval for transmission oforthogonal frequency-division multiplexing (OFDM) symbols on a downlink(DL) channel, the method comprising: receiving, from a transmittingdevice on an uplink (UL) channel, a fine timing measurement (FTM)request frame indicating a number of channel delay spread (CDS)capabilities of the transmitting device; transmitting, to thetransmitting device on the DL channel, a first FTM frame indicating CDScapabilities of the receiving device; receiving, from the transmittingdevice on the UL channel, a first acknowledgement (ACK) frame;estimating a CDS value of the UL channel based on the first ACK frame;transmitting, to the transmitting device on the DL channel, a second FTMframe indicating the estimated CDS value of the UL channel; receiving,from the transmitting device on the UL channel, an FTM feedback frameindicating an estimated CDS value of the DL channel; and selecting avalue for the guard interval based on the estimated CDS value of the DLchannel.
 18. The method of claim 17, wherein the FTM request frameincludes a vendor-specific information element (VSIE) indicating the CDScapabilities of the transmitting device.
 19. The method of claim 18,wherein the VSIE includes a dedicated bit indicating whether thetransmitting device is capable of estimating CDS information for the DLchannel or whether the transmitting device is capable of transmittingthe FTM feedback frame to the receiving device.
 20. The method of claim17, wherein the first FTM frame includes a vendor-specific informationelement (VSIE) indicating the CDS capabilities of the receiving device.21. The method of claim 20, wherein the VSIE includes a dedicated bitindicating whether the receiving device is capable of estimating CDSinformation for the UL channel or whether the receiving device is toreport estimated CDS information for the UL channel to the transmittingdevice.
 22. The method of claim 17, wherein the estimated CDS value ofthe UL channel is included in a first of a time of arrival (TOA) fieldof the second FTM frame and a time of departure (TOD) field of thesecond FTM frame, and a time value indicating a difference between theTOD of the first FTM frame and the TOA of the first ACK frame isincluded in a second of the TOA field of the second FTM frame and theTOD field of the second FTM frame.
 23. The method of claim 17, whereinthe estimated CDS value of the UL channel is included in a time ofarrival (TOA) error field of the second FTM frame or in a time ofdeparture (TOD) error field of the second FTM frame.
 24. The method ofclaim 17, further comprising: transmitting, to the transmitting deviceon the DL channel, a number of OFDM symbols separated by the selectedguard interval.
 25. An apparatus for selecting a guard interval fortransmission of orthogonal frequency-division multiplexing (OFDM)symbols on a downlink (DL) channel, comprising: one or more processors;and a memory comprising instructions that, when executed by the one ormore processors, causes the apparatus to: receive, from a transmittingdevice on an uplink (UL) channel, a fine timing measurement (FTM)request frame indicating a number of channel delay spread (CDS)capabilities of the transmitting device; transmit, to the transmittingdevice on the DL channel, a first FTM frame indicating CDS capabilitiesof the receiving device; receive, from the transmitting device on the ULchannel, a first acknowledgement (ACK) frame; estimate a CDS value ofthe UL channel based on the first ACK frame; transmit, to thetransmitting device on the DL channel, a second FTM frame indicating theestimated CDS value of the UL channel; receive, from the transmittingdevice on the UL channel, an FTM feedback frame indicating an estimatedCDS value of the DL channel; and select a value for the guard intervalbased on the estimated CDS value of the DL channel.
 26. The apparatus ofclaim 25, wherein the FTM request frame includes a vendor-specificinformation element (VSIE) indicating the CDS capabilities of thetransmitting device.
 27. The apparatus of claim 25, wherein the firstFTM frame includes a vendor-specific information element (VSIE)indicating the CDS capabilities of the receiving device.
 28. Theapparatus of claim 25, wherein the estimated CDS value of the UL channelis included in a first of a time of arrival (TOA) field of the secondFTM frame and a time of departure (TOD) field of the second FTM frame,and a time value indicating a difference between the TOD of the firstFTM frame and the TOA of the first ACK frame is included in a second ofthe TOA field of the second FTM frame and the TOD field of the secondFTM frame.
 29. The apparatus of claim 25, wherein the estimated CDSvalue of the UL channel is included in a time of arrival (TOA) errorfield of the second FTM frame or in a time of departure (TOD) errorfield of the second FTM frame.
 30. The apparatus of claim 25, whereinexecution of the instructions causes the apparatus to further: transmit,to the transmitting device on the DL channel, a number of OFDM symbolsseparated by the selected guard interval.